Controller for estimating relative humidity and condensed water, and method for controlling condensed water drain using the same

ABSTRACT

The present invention provides a relative humidity and condensed water estimator for a fuel cell and a method for controlling condensed water drain using the same. Here, the relative humidity and condensed water estimator is utilized in control of the fuel cell system involving control of anode condensed water drain by outputting at least two of signals comprising air-side relative humidity, hydrogen-side relative humidity, air-side instantaneous or cumulative condensed water, hydrogen-side instantaneous or cumulative condensed water, instantaneous and cumulative condensed water of the humidifier, membrane water contents, catalyst layer oxygen partial pressure, catalyst layer hydrogen partial pressure, stack or cell voltage, air-side catalyst layer relative humidity, hydrogen-side catalyst layer relative humidity, oxygen supercharging ratio, hydrogen supercharging ratio, residual water in a stack, and residual water in a humidifier.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 15/097,669, filed Apr. 13, 2016, which is a divisional applicationof U.S. application Ser. No. 13/153,823, filed Jun. 6, 2011 which claimsunder 35 U.S.C. § 119(a) the benefit of Korean Patent Application No.10-2010-0125324 filed Dec. 9, 2010, the entire contents of which areincorporated herein by reference.

BACKGROUND (a) Technical Field

The present invention relates to a relative humidity and condensed waterestimator for a fuel cell, and a method for controlling condensed waterdrain using the same. More particularly, it relates to an estimator fordynamically estimating relative humidity and condensed water in a fuelcell system, and a method for controlling condensed water drain in ananode of a fuel cell stack using the same.

(b) Background Art

Theoretically, a fuel cell system is a simple system that receiveshydrogen and air from the outside to generate electricity and water in astack. In reality, however, water that is a by-product ofelectrochemical reactions is changed into a combination of water vapor,saturated liquid, and ice according to the real-time operationconditions such as temperature and pressure. Since these phase changesaffect the transfer characteristics of water, and also the transfercharacteristics of gases and electrons passing through a gas diffusionlayer, a catalyst layer, a membrane, and a separator channel of a stack,it is hard to estimate internal phenomena of the fuel cell system.

Particularly, since the fuel cell system is a system having highnon-linearity in which the performance of a stack changes due tocoexistence of, so-called, flooding and dry-out phenomena that signifyoverflow and deficiency of water, respectively, it is more difficult toestimate the internal phenomena of the fuel cell system.

For this reason, a sensor for measuring the relative humidity in thefuel cell system and a condensed water level sensor for sensing thelevel of condensed water are installed to manage water in the stack.However, when the relative humidity sensor is used, the cost ofmaterials increases. Also, since the relative humidity sensor and thecondensed water level sensor easily break down due to frequent contactwith water, the maintenance costs may increase, and the reliability maybe reduced in terms of control.

Accordingly, there is a need for the development of a technique fordetermining the relative humidity and the state of condensed water tomanage water in a fuel cell system so as to overcome the abovelimitations.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the invention andtherefore it may contain information that does not form the prior artthat is already known in this country to a person of ordinary skill inthe art.

SUMMARY OF THE DISCLOSURE

The present invention provides an estimator for dynamically estimatingrelative humidity (RH) and condensed water in a fuel cell system, and amethod for controlling condensed water drain in an anode using the same,in order to overcome the flooding phenomenon that signifies overflow ofwater, and the dry-out phenomenon that signifies deficiency of water,which are caused by changes of the amount and the state of transferredwater according to changes of operation conditions such as operationtemperature and pressure of a fuel cell.

In one aspect, the present invention provides a relative humidity andcondensed water estimator for a fuel cell, including controllers fordynamically estimating relative humidity and condensed water in a fuelcell system using fluid dynamics and a mass balance equation of oxygen,nitrogen, hydrogen, and water, wherein the relative humidity andcondensed water estimator is utilized in control of the fuel cell systeminvolving control of anode condensed water drain by outputting at leasttwo of signals including: (1) air-side relative humidity; (2)hydrogen-side relative humidity; (3) air-side instantaneous orcumulative condensed water; (4) hydrogen-side instantaneous orcumulative condensed water; (5) instantaneous and cumulative condensedwater of the humidifier; (6) membrane water contents; (7) catalyst layeroxygen partial pressure; (8) catalyst layer hydrogen partial pressure;(9) stack or cell voltage; (10) air-side catalyst layer relativehumidity; (11) hydrogen-side catalyst layer relative humidity; (12)oxygen supercharging ratio; (13) hydrogen supercharging ratio; (14)residual water in a stack; and (15) residual water in a humidifier.

In one embodiment, the controllers may be mounted in the estimator, andmay include an air blower controller that calculates inflow and relativehumidity and a humidifier controller, The humidifier controller includesa tube control unit that calculates the tube outflow of the humidifierand then calculates air and water balance through proportional-integral(PI) control for estimating a target pressure (P1). The humidifiercontroller can also include a shell control unit that calculates theshell outflow of the humidifier and then calculates air and waterbalance through PI control for estimating a target pressure (P2).

Additionally, the estimator may also have a stack controller thereinthat includes a cathode gas channel (CGC) control unit which calculatesthe outflow of a cathode gas channel of the stack through PI control forestimating a target pressure (P3), and then calculates the air and waterbalance. In addition to the CGC control unit, the stack controller mayalso have a cathode gas diffusion layer (CGDL) control unit, a cathodecatalyst layer (CCL) control unit, a membrane layer (MEM) control unit,an anode catalyst layer (ACL) control unit and an anode gas channel(AGC) control unit. The CGDL control unit calculates the movement of airand water by diffusion and capillary phenomena of a gas diffusion layerby calculating the concentration of air and water. The CCL control unitcalculates generated water a voltage (parameter: current, temperature,oxygen partial pressure, and hydrogen partial pressure) and residualwater through an electrochemical reaction. The MEM control unitcalculates the water concentration of a membrane by osmotic drag, backdiffusion, and heat pipe, and calculates the amount of water moved tothe cathode and anode catalyst layers. Finally, the ACL control unitcalculates the residual water of the anode catalyst layer, the AGDLcontrol unit calculates the movement of air and water by diffusion andcapillary phenomena of the gas diffusion layer by calculating theconcentration of hydrogen and water, and the AGC control unit calculatesthe outflow of an anode gas channel of the stack through PI control forestimating a target pressure (P5), and then calculates the air and waterbalance.

The estimator may also include a fuel processing system (FPS) controller40 which has a hydrogen supply control unit 42 for calculating ahydrogen inflow through PI control for estimating a target pressure P4,a hydrogen inlet manifold control unit 44 for controlling a mixtureratio between supplied hydrogen and recycled hydrogen, a hydrogen outletmanifold control unit 46 for performing hydrogen purging and condensedwater drain control, and a hydrogen recycle loop control unit 48 forcontrolling an ejector and a recycle blower.

In another aspect, the present invention also provides a method forcontrolling condensed water drain using a relative humidity andcondensed water estimator for a fuel cell. This method starts bycalculating a residual amount of condensed water based on a waterbalance equation in an anode condensed water collector ant thendetermining, as a warning stage of a condensed water level sensor, ifthe residual amount of condensed water is greater than a product of adensity (ρ[kg/m{circumflex over ( )}3]) of condensed water and a totalvolume (V1[m{circumflex over ( )}3]) of the condensed water collector,and a duration thereof is greater than the reference value t1. Then thesystem determines that a condensed water level sensor has failed ifV_cell is smaller than V_cell_TH, ΔV_cell is greater than ΔV_cell_TH, ahydrogen recycle blower RPM is smaller than RPM_cmd_RPM_TH, an anodestack inlet pressure is greater than inlet normal pressure map plusP_TH, or an anode stack outlet pressure is greater than outlet normalpressure map plus P_TH, and a duration thereof is greater than areference value t2. In response, the condensed water drain valve iscontrolled based on an anode water trap (AWT) estimated value.

In still another aspect, the present invention provides a method forcontrolling condensed water drain using a relative humidity andcondensed water estimator for a fuel cell. More specifically, a residualamount of condensed water is calculated based on a water balanceequation in an anode condensed water collector and a condensed waterlevel sensor is determined to be in a warning stage if the residualamount of condensed water is equal to or smaller than about 0, and aduration thereof is greater than a reference value (t5). The condensedwater level sensor is determined to have failed if a calculated hydrogenutilization rate is smaller than a value obtained by subtracting ahydrogen utilization rate acceptable reference value from a normalhydrogen utilization rate map, or a hydrogen leakage sensor is on, andthe duration is greater than a reference value (t6). In response tothese determinations, a condensed water drain valve is controlled basedon an anode water trap (AWT) estimated value.

In a further aspect, the present invention provides a method forcontrolling condensed water drain using a relative humidity andcondensed water estimator for a fuel cell. In this method, a cumulativeamount of condensed water, a cumulative amount of generated water, and acumulative condensed water estimated value are calculated based on anoperation of a drain valve in an anode condensed water collector. Acondensed water level sensor is determined to be in a warning stage if acumulative condensed water ratio (AWT_ratio1) is smaller than a valueobtained by subtracting a cumulative condensed water ratio differenceacceptable reference value (AWT_TH) from a cumulative condensed waterratio (AWT_ratio2), and a duration thereof is greater than a referencevalue (t1). However, the condensed water level sensor is determined tohave failed if V_cell is smaller than V_cell_TH, ΔV_cell is greater thanΔV_cell_TH, hydrogen recycle blower RPM is smaller than RPM_cmd_RPM_TH,anode stack inlet pressure is greater than inlet normal pressure mapplus P_TH, or anode stack outlet pressure is greater than outlet normalpressure map plus P_TH, and a duration there is greater than a referencevalue (t2). In response to these determinations, a condensed water drainvalve is controlled based on an anode water trap (AWT) estimated value.

In a still further aspect, the present invention provides a method forcontrolling condensed water drain using a relative humidity andcondensed water estimator for a fuel cell. In this method, a cumulativeamount of condensed water, a cumulative amount of generated water, and acumulative condensed water estimated value are calculated based on anoperation of a drain valve in an anode condensed water collector. Acondensed water level sensor is then determined to be in a warning stageif a cumulative condensed water ratio (AWT_ratio1) is greater than avalue obtained by adding a cumulative condensed water ratio differenceacceptable reference value (AWT_TH) to a cumulative condensed waterratio (AWT_ratio2), and a duration thereof is greater than a referencevalue (t5). However, the condensed water level sensor is determined tohave failed if a calculated hydrogen utilization rate is smaller than avalue obtained by subtracting a hydrogen utilization rate acceptablereference value H2_Util_TH from a normal hydrogen utilization rate map,or a hydrogen leakage sensor is on, and a duration thereof is greaterthan a reference value (t6). In response to these determinations, acondensed water drain valve is controlled based on an anode water trap(AWT) estimated value.

Other aspects and preferred embodiments of the invention are discussedinfra.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will now bedescribed in detail with reference to certain exemplary embodimentsthereof illustrated the accompanying drawings which are givenhereinbelow by way of illustration only, and thus are not limitative ofthe present invention, and wherein:

FIG. 1 is a block diagram illustrating representative input and outputsignals of a relative humidity and condensed water estimator for a fuelcell according to an exemplary embodiment of the preset invention;

FIG. 2 is a diagram illustrating a relative humidity and condensed waterestimator for a fuel cell according to an exemplary embodiment of thepresent invention;

FIG. 3 is a block diagram illustrating an air blower controller of arelative humidity and condensed water estimator for a fuel cellaccording to an exemplary embodiment of the present invention;

FIGS. 4A and 4B are block diagrams illustrating a humidifier controllerof a relative humidity and condensed water estimator for a fuel cellaccording to an exemplary embodiment of the present invention;

FIGS. 5A through 5G are block diagrams illustrating a stack controllerof a relative humidity and condensed water estimator for a fuel cellaccording to an exemplary embodiment of the present invention;

FIGS. 6A and 6B are block diagrams illustrating a fuel processing systemcontroller of a relative humidity and condensed water estimator for afuel cell according to an exemplary embodiment of the present invention;

FIGS. 7A and 7B are flowcharts illustrating methods for controllingcondensed water drain using a relative humidity and condenser waterestimator for a fuel cell according first and second exemplaryembodiments of the present invention; and

FIGS. 8A and 8B are flowcharts illustrating methods for controllingcondensed water drain using a relative humidity and condenser waterestimator for a fuel cell according third and fourth exemplaryembodiments of the present invention.

Reference numerals set forth in the Drawings includes reference to thefollowing elements as further discussed below:

10: relative humidity and condensed water estimator

20: internal estimator air blower controller

30: internal estimator humidifier controller

40: internal estimator fuel processing system (FPS) controller

50: internal estimator stack controller

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variouspreferred features illustrative of the basic principles of theinvention. The specific design features of the present invention asdisclosed herein, including, for example, specific dimensions,orientations, locations, and shapes will be determined in part by theparticular intended application and use environment.

In the figures, reference numbers refer to the same or equivalent partsof the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Hereinafter reference will now be made in detail to various embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings and described below. While the invention will bedescribed in conjunction with exemplary embodiments, it will beunderstood that present description is not intended to limit theinvention to those exemplary embodiments. On the contrary, the inventionis intended to cover not only the exemplary embodiments, but alsovarious alternatives, modifications, equivalents and other embodiments,which may be included within the spirit and scope of the invention asdefined by the appended claims.

It is understood that the term “vehicle” or “vehicular” or other similarterm as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g. fuels derived fromresources other than petroleum). As referred to herein, a hybrid vehicleis a vehicle that has two or more sources of power, for example bothgasoline-powered and electric-powered vehicles.

Hereinafter, exemplary embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.

The present invention relates to an estimator for dynamically estimatingrelative humidity and condensed water of a fuel cell system, and amethod for controlling condensed water drain existing in an anode usingthe estimator.

The estimator for estimating relative humidity and condensed water ofthe fuel cell system is a kind of a controller that dynamicallyestimates the relative humidity and the condensed water in the fuel cellsystem, based on temperature/flow rate/pressure sensor signals ofair/hydrogen/cooling water for the purpose of overall control of thefuel cell system. The output signal of the estimator may includeair-side/hydrogen-side relative humidity,air-side/hydrogen-side/humidifier instantaneous and cumulative condensedwater ratio, membrane water content, catalyst layer oxygen/hydrogenpartial pressure, stack voltage, air-side/hydrogen-side catalyst layerrelative humidity, oxygen/hydrogen supercharging ratio, residual waterin a stack, and residual water in a humidifier.

As shown in FIG. 1, the relative humidity and condensed water estimatoraccording to an embodiment of the present invention may receive inputsignals including ambient temperature, atmospheric pressure, atmosphericrelative humidity (RH), stack current, flow rate of air supplied to acathode of a stack, air temperature at the inlet/outlet of thehumidifier and the inlet/outlet of the stack, air pressure at theinlet/outlet of the humidifier and the inlet/outlet of the stack,cooling water temperature at the inlet/output of the stack, flow rate ofhydrogen supplied to the anode of the stack, hydrogen temperature at thehydrogen supply line and the inlet/outlet of the stack, hydrogenpressure at the inlet/outlet of the stack, and an actuator signalincluding hydrogen blower speed and on/off state of hydrogen purge/drainvalve.

The relative humidity and condensed water estimator may output signalsincluding cathode relative humidity (RH) at the inlet/outlet of thestack and the inlet/outlet of the humidifier, anode relative humidity(RH) at the inlet/outlet of the stack, instantaneous and cumulativecondensed water ratio of cathode/anode/humidifier (based on CWT, AWT,and SWT), membrane water content, oxygen/hydrogen partial pressure ofcathode and anode catalyst layers, stack voltage, relative humidity ofthe cathode/anode catalyst layer, oxygen/hydrogen supercharging ratio,residual water of the stack (for each layer), and residual water of thehumidifier (for each tube/shell).

The relative humidity and condensed water estimator may output at leasttwo of signals including: (1) air-side relative humidity; (2)hydrogen-side relative humidity; (3) air-side instantaneous orcumulative condensed water; (4) hydrogen-side instantaneous orcumulative condensed water; (5) instantaneous and cumulative condensedwater of the humidifier; (6) membrane water contents; (7) catalyst layeroxygen partial pressure; (8) catalyst layer hydrogen partial pressure;(9) stack or cell voltage; (10) air-side catalyst layer relativehumidity; (11) hydrogen-side catalyst layer relative humidity; (12)oxygen supercharging ratio; (13) hydrogen supercharging ratio; (14)residual water in the stack; and (15) residual water in the humidifier,in order to dynamically estimate the relative humidity and the condensedwater in the fuel cell system using the fluid dynamics and mass balanceequation of oxygen, nitrogen, hydrogen, and water.

For reference, portions indicated as ovals in each drawing representoutput signals of the relative humidity and condensed water estimator.

As shown in FIG. 2, the estimator 10 that receives and outputs signalsas described above illustratively includes an internal estimator airblower controller 20, an internal estimator humidifier controller 30including a tube control unit 32 and a shell control unit 34, aninternal estimator fuel processing system (FPS) controller 40, and aninternal estimator stack controller 50.

As shown in FIG. 3, the air blower controller 20, which calculates theinflow and the relative humidity, may calculate the cathode relativehumidity at the inlet of the humidifier on the assumption that a vaporflow supplied through an air blower is conserved. That is, the airblower controller 20 may receive ambient temperature, atmosphericpressure, and atmospheric relative humidity (using a sensor signal or anexperimental map) to calculate the cathode relative humidity at theinlet of the humidifier and send the calculated relative humidity to thehumidifier controller 30 as a signal.

The humidifier illustratively includes a shell receiving humid airdischarged from the cathode of the stack and a tube (e.g., a hollowfiber polymeric membrane) that is a passage allowing dry air to flowfrom the air blower to the cathode of the stack and also receives thehumid air from the shell to humidify air. The humidifier controller 30for overall control of fluid flowing in the humidifier illustrativelyincludes the tube control unit 32 which calculates the tube outflow ofthe humidifier and then calculates air and water balance throughproportional-integral (PI) control for estimating a target pressure P1,and the shell control unit 34 that calculates the shell outflow of thehumidifier and then calculates air and water balance through PI controlfor estimating a target pressure P2.

As shown in FIG. 4A, the tube control unit 32 of the humidifiercontroller 30 illustratively includes a dry air balance calculation unit32-1, a water balance calculation unit 32-2, and an outflow calculationunit 32-3. The dry air balance calculation unit 32-1 illustrativelyincludes an integrator that receives a dry air inflow at the inlet ofthe humidifier, air temperature at the cathode inlet of the stack and adry air outflow which is bypassing the humidifier tube without beinghumidified, and that calculates a dry air partial pressure at thehumidifier tube. The water balance calculation unit 32-2 illustrativelyincludes an integrator that receives a vapor inflow at the inlet of thehumidifier, a vapor outflow from the humidifier shell at the inlet ofthe stack, air temperature at the inlet of the stack, and thatcalculates a vapor partial pressure at the humidifier tube, cathoderelative humidity at the inlet of the stack, and a residual water of thehumidifier tube. The outflow calculation unit 32-3 may calculate a dryair outflow at the inlet of the stack and a vapor outflow at the inletof the stack, based on the air pressure and the target pressure P1 atthe inlet of the stack.

In this case, the vapor outflow from the humidifier shell may becalculated using a diffusion equation by a difference between wateractivities of the humidifier tube and shell. The outflow, that is, thedry air outflow at the inlet of the stack and the vapor outflow at theinlet of the stack, may be calculated by applying PI control forestimating the air pressure (at the inlet of the stack) as the targetpressure P1. The target pressure PI may utilize air pressure (stackinlet) sensor data or air flow-based map data.

As shown in FIG. 4B, the shell control unit 34 of the humidifiercontroller 30 illustratively includes a dry air balance calculation unit34-1, a water balance calculation unit 34-2, and an outflow calculationunit 34-3. The dry air balance calculation unit 34-1 illustrativelyincludes an integrator that receives a dry air inflow at the outlet ofthe stack, a dry air outflow at the outlet of the humidifier, and airtemperature at the outlet of the humidifier, and that calculates a dryair partial pressure at the humidifier shell. The water balancecalculation unit 34-2 illustratively includes an integrator thatreceives a vapor inflow at the outlet of the stack, a vapor outflow atthe outlet of the humidifier, a vapor delivery flow delivered to thehumidifier tube, a liquid inflow at the outlet of the stack, a liquidoutflow at the outlet of the humidifier, and air temperature at theoutlet of the humidifier, and that calculates a vapor partial pressureat the humidifier shell, cathode relative humidity at the outlet of thehumidifier, and a residual water of the humidifier shell. The outflowcalculation unit 34-3 may calculate a dry air outflow at the outlet ofthe humidifier and a vapor outflow at the outlet of the humidifier,based on the air pressure and the target pressure P2 at the outlet ofthe humidifier.

In this case, the vapor delivery flow delivered to the humidifier tubemay be identical to the vapor delivery flow delivered from thehumidifier shell. The outflow, that is, the dry air outflow at theoutlet of the humidifier and the vapor outflow at the outlet of thehumidifier, may be calculated by applying PI control for estimating theair pressure (at the outlet of the humidifier) as the target pressureP2. Also, the target pressure P2 may utilize air pressure (at the outletof the humidifier) sensor data or air flow-based map data, and the airtemperature (at the outlet of the humidifier) may utilize valuescalculated based on the energy balance equation, using sensor data orair temperature sensor value at the inlet of the humidifier and theinlet/outlet of the stack

On the other hand, the liquid inflow at the outlet of the stack maysignify a flow that is trapped in the cathode water trap (CWT), and theliquid outflow at the outlet of the humidifier may signify a flow thatis trapped in the humidifier shell water trap (SWT). If the cathoderelative humidity at the outlet of the humidifier is smaller than about100%, shell water trap (SWT)=cathode water trap (CWT)×α, where α rangesfrom about 0 to about 1.

Alternatively, if the cathode relative humidity at the outlet of thehumidifier is equal to or greater than about 100%, shell water trap(SWT)=cathode water trap (CWT)×α+humidifier shell net water flow×β,where α ranges from about 0 to about 1, and β ranges from about 0 toabout 1.

Here, the stack controller 50, as shown in FIG. 2, illustrativelyincludes a cathode gas channel (CGC) control unit 51, a cathode gasdiffusion layer (CGDL) control unit 52, a cathode catalyst layer (CCL)control unit 53, a membrane layer (MEM) control unit 54, an anodecatalyst layer (ACL) control unit 55, an anode gas diffusion layer(AGDL) control unit 56 and an anode gas channel (AGC) control unit 57.The CGC control unit 51 calculates the outflow of a cathode gas channelof the stack through PI control for estimating a target pressure P3, andthen calculates the air and water balance. The CGDL control unit 52calculates the movement of air and water by diffusion and capillaryphenomena of the gas diffusion layer by calculating the concentration ofair and water. The CCL control unit 53 calculates generated water andcalculates a voltage (parameter: current, temperature, oxygen partialpressure, and hydrogen partial pressure) and residual water through anelectrochemical reaction. The MEM control unit 54 calculates the waterconcentration of a membrane by osmotic drag, back diffusion, and heatpipe, and calculates the amount of water moved to the cathode and anodecatalyst layers. The ACL control unit 55 calculates the residual waterof the anode catalyst layer. The AGDL control unit 56 calculates themovement of air and water by diffusion and capillary phenomena of thegas diffusion layer by calculating the concentration of hydrogen andwater. Finally, the AGC control unit 57 calculates the outflow of ananode gas channel of the stack through PI control for estimating atarget pressure P5, and calculate the air and water balance.

As shown in FIG. 5A, the cathode gas channel (CGC) control unit 51 ofthe stack controller 50 illustratively includes a dry air balancecalculation unit 51-1 including an integrator receiving dry air inflowat the inlet of the stack, dry air outflow at the outlet of the stack,oxygen outflow with respect to the cathode gas diffusion layer (CGDL),and air temperature at the outlet of the stack to calculate oxygenovercharging ratio, oxygen concentration of the cathode gas channel, anddry air partial pressure of the cathode gas channel. The stack CGCcontrol unit 51 also illustratively includes a water balance calculationunit 51-2 and an outflow calculation unit 51-3. More specifically, thewater balance calculation unit 51-2 illustratively includes anintegrator receiving vapor inflow at the inlet of the stack, vaporoutflow at the outlet of the stack, vapor flow from the cathode gasdiffusion layer (CGDL), liquid outflow at the outlet of the stack,liquid flow from the cathode gas diffusion layer, and air temperature atthe outlet of the stack to calculate vapor partial pressure of thecathode gas channel, cathode relative humidity at the outlet of thestack, residual water of the cathode gas channel, vapor concentration ofthe cathode gas channel, and liquid S (CGC Liquid S=(s−s_im)/(1×s_im)).The outflow calculation unit 51-3 calculates a dry air outflow at theoutlet of the stack and vapor outflow at the outlet of the stack, basedon the target pressure P3 and the air pressure of the cathode gaschannel (CGC).

In this case, the outflow, that is, the dry air outflow at the outlet ofthe stack and the vapor outflow at the outlet of the stack, may becalculated by applying PI control for estimating the air pressure (atthe outlet of the stack) as the target pressure P3. Also, the targetpressure P3 may utilize air pressure (at the outlet of the stack) sensordata and air flow-based map data, and the air temperature (at the outletof the stack) may utilize sensor data or stack cooling waterinlet/outlet temperature-based map data.

The liquid outflow of the outlet of the stack is a flow that is trappedin the cathode water trap (CWT). If the cathode relative humidity of theoutlet of the stack is smaller than about 100%, cathode water trap(CWT)=liquid flow from cathode gas diffusion layer×α, where α rangesfrom about 0 to about 1.

If the cathode relative humidity of the outlet of the stack is greaterthan about 100%, cathode water trap (CWT)=liquid flow from cathode gasdiffusion layer×α+cathode gas channel (CGC) shell net water flow×β,where α ranges from about 0 to about 1, and β ranges from about 0 toabout 1.

If s>s_im, cathode gas channel liquid S (CGC LiquidS)=(s−s_im)/(1−s_im). If s≤s_im, S (CGC Liquid S)=0. Here, s=residualliquid volume of cathode gas channel divided by the volume of cathodegas channel, and s_im signifies immobile saturation that is a referencecondition in which liquid flow is generated in the capillary phenomenon.

As shown in FIG. 5B, the cathode gas diffusion layer (CGDL) control unit52 of the stack controller 50 illustratively includes an oxygen balancecontrol unit 52-1, a vapor balance control unit 52-2, a liquid balancecontrol unit 52-3, a condensation calculation unit 52-4, a liquid flowcalculation unit 52-5, an oxygen diffusion calculation unit 52-6 and avapor diffusion calculation unit 52-7. The vapor balance control unit52-2 includes an integrator that calculates the oxygen concentration ofthe cathode gas diffusion layer (CGDL), based on oxygen inflow from thecathode gas channel and oxygen outflow to the cathode catalyst layer(CCL). The vapor balance control unit 52-2 includes an integrator thatcalculates the residual amount of vapor of the cathode gas diffusionlayer (CGDL), based on vapor inflow from the cathode catalyst layer(CCL) and vapor outflow to the cathode gas channel (CGC). Additionally,the liquid balance control unit 52-3 includes an integrator calculatingthe residual amount of liquid of the cathode gas diffusion layer (CGDL),based on condensation rate of the cathode gas diffusion layer and liquidflow to the cathode gas channel Furthermore, the condensationcalculation unit 52-4 calculates the condensation rate of cathode gasdiffusion layer (CGDL) based on temperature and vapor concentration ofthe cathode gas diffusion layer (CGDL) and the liquid flow calculationunit 52-5 calculates liquid flow of the cathode gas channel based onliquid S (CGDL Liquid S=(s−s_im)/(1−s_im)) and temperature of thecathode gas diffusion layer, and cathode gas channel liquid S (CGCLiquid S), an oxygen diffusion calculation unit 52-6 calculates oxygeninflow from the cathode gas channel based on oxygen concentration of thecathode gas diffusion layer, oxygen concentration of the cathode gaschannel, and oxygen diffusion coefficient of the cathode gas diffusionlayer. Finally, the vapor diffusion calculation unit 52-7 calculatesvapor outflow to the cathode gas channel based on vapor concentration ofthe cathode gas diffusion layer, vapor concentration of the cathode gaschannel, and vapor diffusion coefficient of the cathode gas diffusionlayer.

In this case, the temperature of the cathode gas diffusion layer (CGDL)may utilize stack cooling water inlet/outlet temperature-based map data.

If s>s_im, cathode gas diffusion layer liquid S (CGDL LiquidS)=(s−s_im)/(1−s_im). If s≤s_im, S (CGDL Liquid S)=0. Here, s equalsresidual liquid volume of cathode gas diffusion layer (CGDL) divided bythe pore volume of cathode gas diffusion layer (CGDL), and s_imsignifies immobile saturation that is a reference condition in whichliquid flow is generated in the capillary phenomenon. Under thecapillary phenomenon, if s>s_im, liquid flow may be generated by adifference of liquid S of adjacent layers.

On the other hand, the condensation rate of the cathode gas diffusionlayer (CGDL) may be generated in proportion to a difference betweensaturation pressure according to the temperature of the cathode gasdiffusion layer and the vapor pressure according to the concentration ofthe vapor of the cathode gas diffusion layer (CGDL vapor). Also, asliquid of the cathode gas diffusion layer (CGDL) increases (that is,CGDL liquid s increases), the diffusion coefficient of oxygen and vapormay be reduced.

As shown in FIG. 5C, the cathode catalyst layer (CCL) control unit 53 ofthe stack controller 50 illustratively includes an oxygen partialpressure calculation unit 53-1, a relative humidity calculation unit53-2, a residual vapor calculation unit 53-3, a vapor flow calculationunit 53-4 and a voltage calculation unit 53-5. The oxygen partialpressure calculation unit 53-1 calculates an oxygen partial pressure ofthe cathode catalyst layer (CCL) based on the oxygen concentration ofthe cathode gas diffusion layer (CGDL) and the temperature of thecathode catalyst layer (CCL). The relative humidity calculation unit53-2 calculates the relative humidity of the cathode catalyst layer(CCL) based on the temperature of the cathode catalyst layer (CCL). Theresidual vapor calculation unit 53-3 calculates the residual amount ofvapor of the cathode catalyst layer (CCL) based on the vaporconcentration of the cathode gas diffusion layer (CGDL). The vapor flowcalculation unit 53-4 calculates the vapor flow to the cathode gasdiffusion layer based on the generated water flow by the electrochemicalreaction of the stack and the vapor flow to the membrane. Lastly, thevoltage calculation unit 53-5 calculates a voltage by theelectrochemical reaction of the stack based on the stack current, thetemperature of the cathode catalyst layer, the electrical resistance ofthe membrane, the oxygen partial pressure of the cathode catalyst layer,and the hydrogen partial pressure of the anode catalyst layer.

As shown in FIG. 5D, the membrane layer (MEM) control unit 54 of thestack controller 50 illustratively includes an electro-osmotic dragdetection unit 54-1, back-diffusion detection unit 54-2, a heat pipedetection unit 54-3 and a water balance calculation unit 54-4 outputtingelectro-osmotic drag of the cathode catalyst layer and the anodecatalyst layer based on working current and water content of themembrane. The back-diffusion detection unit 54-2 detects back-diffusionof the cathode catalyst layer and the anode catalyst layer based on therelative humidity of the cathode catalyst layer, the relative humidityof the anode catalyst layer, and the temperature and water content ofthe membrane. The heat pipe detection unit 54-3 detects water movementstate of a heat pipe mechanism based on the temperature of the membrane,a temperature difference between the cathode catalyst layer and thecathode gas diffusion layer, and a temperature difference between thecathode catalyst layer and the anode gas diffusion layer. Lastly, thewater balance calculation unit 54-4 includes an integrator that receivesan electro-osmotic drag output signal of the electro-osmotic dragdetection unit 54-1 and a back-diffusion output signal of theback-diffusion detection unit 54-2, an output signal of the heat pipedetection unit 54-3, and the temperature of the membrane to calculatethe residual water, electrical resistance, and water content of themembrane.

In the membrane layer (MEM) control unit 54, the temperature of themembrane (MEM) may utilize the stack cooling water inlet/outlettemperature-based map data. The temperature difference between thecathode catalyst layer (CCL) and the cathode gas diffusion layer (CGDL),the temperature difference between the cathode catalyst layer (CCL) andthe anode gas diffusion layer (AGDL) may utilize the working current andthe stack cooling water inlet/outlet temperature-based map data, and thevapor flow from the cathode catalyst layer and the vapor flow to theanode catalyst layer may be calculated by summing the electro-osmoticdrag, back-diffusion rate, and heat pipe output values of the cathodecatalyst layer and the anode catalyst layer, respectively.

On the other hand, the electro-osmotic drag water movement mechanismrefers to water movement in which hydrogen ions (H+) pass through themembrane according the working current, and the back diffusion watermovement mechanism refers to water movement according to water activityof the cathode catalyst layer and the anode catalyst layer of the bothends of the membrane. The heat-pipe water movement mechanism refers towater movement (from hot layer to cold layer) by temperature gradientbetween layers in a saturated state of the membrane.

Also, the membrane water content, which is a representative factor fordetermining the membrane dry-out and the flooding, is a dimensionlessfactor between about 0 and about 16.8. As the membrane water contentdecreases, the membrane becomes dried out.

As shown in FIG. 5E, the anode catalyst layer (ACL) control unit 55 ofthe stack controller 50, which calculates the residual water of theanode catalyst layer, illustratively includes a hydrogen partialpressure calculation unit 55-1, a relative humidity calculation unit55-2, a residual vapor calculation unit 55-3 and a vapor flowcalculation unit 55-4. The hydrogen partial pressure calculation unit55-1 calculates the hydrogen partial pressure of the anode catalystlayer (ACL) based on hydrogen concentration of the anode gas diffusionlayer (AGDL) and temperature of the anode catalyst layer (ACL). Therelative humidity calculation unit 55-2 calculates relative humidity ofthe anode catalyst layer (ACL) based on vapor concentration of the anodegas diffusion layer (AGDL) and temperature of the anode catalyst layer(ACL). The residual vapor calculation unit 55-3 calculates the residualamount of vapor of the anode catalyst layer (ACL) based on vaporconcentration of the anode gas diffusion layer (AGDL), and the vaporflow calculation unit 55-4 calculates vapor flow into the anode gasdiffusion layer based on vapor flow from the membrane.

In this case, the temperature of the anode catalyst layer may utilizethe stack cooling water inlet/outlet temperature-based map data.

As shown in FIG. 5F, the anode gas diffusion layer (AGDL) control unit56 of the stack controller 50 illustratively includes an hydrogenbalance control unit 56-1, a vapor balance control unit 56-2, a liquidbalance control unit 56-3, a condensation calculation unit 56-4, and aliquid flow calculation unit 56-5. The hydrogen balance control unit56-1 includes an integrator that calculates the hydrogen concentrationof the anode gas diffusion layer (AGDL), based on hydrogen inflow fromthe anode gas channel (AGC) and hydrogen outflow to the anode catalystlayer (ACL). The vapor balance control unit 56-2 includes an integratorthat calculates the residual water and concentration of vapor of theanode gas diffusion layer (AGDL), based on vapor inflow from the anodecatalyst layer (ACL) and vapor outflow to the anode gas channel (AGC).The liquid balance control unit 56-3 includes an integrator to calculatethe residual amount of liquid of the anode gas diffusion layer (AGDL),based on condensation rate of the anode gas diffusion layer (AGDL) andliquid flow to the anode gas channel (AGC). The condensation calculationunit 56-4 calculates the condensation rate of anode gas diffusion layer(AGDL) based on temperature and vapor concentration of the anode gasdiffusion layer (AGDL). The liquid flow calculation unit 56-5 calculatesliquid flow of the anode gas channel based on liquid S (AGDL LiquidS=(s−s_im)/(1−s_im)) and temperature of the anode gas diffusion layer,and anode gas channel liquid S (AGC Liquid S). The hydrogen diffusioncalculation unit 56-6 calculates hydrogen inflow from the anode gaschannel based on hydrogen concentration of the anode gas diffusionlayer, hydrogen concentration of the anode gas channel, and hydrogendiffusion coefficient of the anode gas diffusion layer. The vapordiffusion calculation unit 56-7 calculates vapor outflow to the anodegas channel based on vapor concentration of the anode gas diffusionlayer, vapor concentration of the anode gas channel, and vapor diffusioncoefficient of the anode gas diffusion layer.

In this case, the temperature of the anode gas diffusion layer (AGDL)may utilize stack cooling water inlet/outlet temperature-based map data.

If s>s_im, anode gas diffusion layer liquid S (AGDL LiquidS)=(s−s_im)/(1−s_im). If s≤s_im, S (AGDL Liquid S)=0. Here, s (AGDLliquid s)=residual liquid volume of anode gas diffusion layer (AGDL)divided by the pore volume of anode gas diffusion layer (AGDL), and s_imsignifies immobile saturation that is a reference condition in whichliquid flow is generated in the capillary phenomenon. Under thecapillary phenomenon, if s>s_im, liquid flow may be generated by adifference of liquid S of adjacent layers.

On the other hand, the condensation rate of the anode gas diffusionlayer (AGDL) may be generated in proportion to a difference betweensaturation pressure according to the temperature of the anode gasdiffusion layer and the vapor pressure according to the concentration ofthe vapor of the anode gas diffusion layer (AGDL vapor). Also, as liquidof the anode gas diffusion layer (AGDL) increases (that is, AGDL liquids increases), the diffusion coefficient of hydrogen and vapor may bereduced.

As shown in FIG. 5G, the anode gas channel (AGD) control unit 57 of thestack controller 50 illustratively includes a hydrogen balancecalculation unit 57-1, a water balance calculation unit 57-2 and anoutflow calculation unit 57-3. The hydrogen balance calculation unit57-1 includes an integrator that receives hydrogen inflow at the inletof the stack, hydrogen outflow at the outlet of the stack, hydrogenoutflow to the anode gas diffusion layer, anode gas channel (AGC)temperature at the outlet of the stack, and hydrogen outflow at theoutlet of the stack to calculate a hydrogen supercharging ratio, ahydrogen concentration of the anode gas channel, and a hydrogen partialpressure of the anode gas channel. The water balance calculation unit57-2 includes an integrator that receives vapor inflow of the inlet ofthe stack, vapor outflow of the outlet of the stack, vapor flow from theanode gas diffusion layer (AGDL), liquid outflow at the outlet of thestack, liquid inflow from the anode gas diffusion layer, and anode gaschannel (AGC) temperature at the outlet of the stack to calculate vaporpartial pressure of the anode gas channel, anode relative humidity atthe outlet of the stack, residual water of the anode gas channel, andvapor concentration and liquid S (AGC Liquid S=(s−s_im)/(1−s_im)) of theanode gas channel. Finally, the outflow calculation unit 57-3 calculateshydrogen outflow and vapor outflow at the outlet of the stack, based onthe target pressure P5 and the anode gas channel (AGC) pressure at theoutlet of the stack.

In this case, the outflow, that is, the hydrogen outflow and the vaporoutflow at the outlet of the stack may be calculated by applying PIcontrol for estimating the anode gas channel pressure (at the outlet ofthe stack) as the target pressure P5. The target pressure P5 may utilizeanode gas channel pressure (at the outlet of the stack) sensor data oranode gas channel flow-based map data. The anode gas channel temperature(at the outlet of the stack) may utilize sensor data or stack coolingwater inlet/outlet temperature-based map data.

The liquid outflow at the outlet of the stack is a flow that is trappedin an anode water trap (AWT). If the anode relative humidity at theoutlet of the stack is smaller than about 100%, anode water trap(AWT)=liquid flow from anode gas diffusion layer×α, where α ranges fromabout 0 to about 1.

On the other hand, if the anode relative humidity at the outlet of thestack is equal to or greater than about 100%, anode water trap(AWT)=liquid flow from anode gas diffusion layer×α+anode gas channel(AGC) net water flow×β, where α ranges from about 0 to about 1, and βranges from about 0 to about 1.

If s>s_im, anode gas channel liquid S (AGC Liquid S)=(s−s_im)/(1−s_im).If s≤s_im, AGC Liquid S=0. Here, s=residual liquid volume of anode gaschannel divided by the volume of anode gas channel, and s_im signifiesimmobile saturation that is a reference condition in which liquid flowis generated in the capillary phenomenon.

As shown in FIGS. 6A and 6B, the fuel processing system (FPS) controller40 illustratively includes a hydrogen supply control unit 42 calculatinga hydrogen inflow through PI control for estimating a target pressureP4, a hydrogen inlet manifold control unit 44 controlling a mixtureratio between supplied hydrogen and recycled hydrogen, a hydrogen outletmanifold control unit 46 for performing hydrogen purging and condensedwater drain control, and a hydrogen recycle loop control unit 48 forcontrolling an ejector and a recycle blower.

More specifically, the fuel processing system controller 40illustratively includes the hydrogen supply control unit 42 thatreceives the target pressure P4, anode gas channel pressure at theoutlet of the stack, hydrogen supply temperature, and relative humidityof supplied hydrogen to output hydrogen inflow, hydrogen supplypressure, hydrogen supply temperature, and relative humidity of suppliedhydrogen to the hydrogen inlet manifold control unit 44. The hydrogeninlet manifold control unit 44 receives hydrogen inflow, hydrogen supplypressure, hydrogen supply temperature and relative humidity of suppliedhydrogen from the hydrogen supply control unit 42, and at the same timereceives hydrogen recycle flow, vapor recycle flow, hydrogen secondrecycle loop temperature, hydrogen second recycle loop pressure, andhydrogen second recycle loop relative humidity from the hydrogen recycleloop control unit 48 to output hydrogen flow at the inlet of the stack,vapor flow at the inlet of the stack, anode gas channel temperature andpressure at the inlet of the stack, and anode relative humidity at theinlet of the stack to the anode gas channel (AGC) control unit 57. Thehydrogen outlet manifold control unit 46 receives hydrogen flow at theoutlet of the stack, vapor flow at the outlet of the stack, liquid flowat the outlet of the stack, anode gas channel temperature and pressureat the outlet of the stack, and anode relative humidity at the outlet ofthe stack from the anode gas channel (AGC) control unit 57 to outputhydrogen recycle flow, vapor recycle flow, hydrogen first recycle looptemperature, hydrogen first recycle loop pressure, hydrogen firstrecycle loop relative humidity to the hydrogen recycle loop controlunit, and the hydrogen recycle loop control unit 48 receiving hydrogenrecycle flow, vapor recycle flow, hydrogen first recycle looptemperature, hydrogen first recycle loop pressure, hydrogen firstrecycle loop relative humidity from the hydrogen outlet manifold controlunit 46 to output hydrogen recycle flow, vapor recycle flow, hydrogensecond recycle loop temperature, hydrogen second recycle loop pressure,hydrogen second recycle loop relative humidity.

In the hydrogen recycle loop, the second recycle looptemperature/pressure/relative humidity may be calculated based on heatmap according to revolutions per minute (RPM) of the hydrogen recycleblower. Hydrogen and vapor purge flow may be calculated from a nozzlecalculation formula by a pressure difference when the hydrogen purgevalue turns on. The condensed water drain flow may be calculated basedon condensed water discharge test map by unit time. The amount ofcondensed water remaining in the condensed drain port may be calculatedfrom the liquid balance equation in consideration of anode water trap(AWT) inputted into condensed water drain flow.

Also, the hydrogen inflow may be calculated by applying PI control forestimating anode gas channel pressure (utilizing stack outlet anode gaschannel pressure-based map) at the inlet of the stack. The targetpressure P4 may utilize anode gas channel pressure (at the inlet of thestack) sensor data or anode gas channel flow-based map data. Thehydrogen supply temperature may utilize sensor data or map data based onambient temperature, hydrogen tank temperature, and stack cooling waterinlet/outlet temperature.

In this case, the relative humidity of supplied hydrogen may be set toabout 0% in consideration of the pure hydrogen condition. The anode gaschannel temperature (at the inlet of the stack) may be calculated fromthe energy balance equation of supplied plus vapor heat and recyclehydrogen plus vapor heat. The anode relative humidity (at the inlet ofthe stack) may be calculated based on a humidity ratio that is definedas a ratio of vapor flow to hydrogen flow.

Hereinafter, a method for controlling condensed water drain using arelative humidity and condensed water estimator for a fuel cell based onthe above-described configuration will be described in detail.

The method for controlling condensed water drain may be achieved using,as indicated using ovals in the drawings, air-side/hydrogen-siderelative humidity, air-side/hydrogen-side/humidifier instantaneous andcumulative condensed water ratio, membrane water content ratio, catalystlayer oxygen/hydrogen partial pressure, stack voltage,air-side/hydro-side catalyst layer relative humidity, oxygen/hydrogensupercharging ratio, residual water in the stack, and residual water inthe humidifier, among output signals of the relative humidity andcondensed water estimator.

The method for controlling anode condensed water drain using therelative humidity and condensed water estimator according to the firstand second embodiments of the present invention may perform the controlof the drain valve on/off based on condensed water estimates uponabnormality of the water level sensor 45 by calculating the residualamount of condensed water in the anode condensed water collector, usingthe amount of anode condensed water calculated in the estimator andon/off signals of the drain valve, and then determining whether thewater level sensor 45 in the condensed water collector 43 as shown inFIG. 6B is normal.

More specifically, the method illustratively includes determiningwhether the water level sensor in the condensed water collector isnormal by calculating the residual amount of condensed water in theanode condensed water collector, based on anode condensed water (inputflow, AWT) calculated in the above estimator and outflow (anodecondensed water drain valve on×condensed water outflow per second)calculated using the drain valve on/off signals, and, if abnormal,performing the drain valve on/off control based on the residualcondensed water estimate.

In this case, the residual amount of condensed water (calculatedvalue=estimated value) is greater than an amount corresponding to thetotal volume V1 of the collector of the anode condensed water, there isa possibility that the drain valve is excessively closed due to afailure of the water level sensor. Thus, if cell voltage drop, abnormaloperation of the hydrogen recycle blower, and abnormality of anode-sidestack inlet/outlet pressure occur due to serious flooding in the stack,it is determined that the water level sensor has failed. Then, condensedwater drain valve may be controlled on/off based on the anode water trap(AWT) estimate to maintain residual condensed water near the water levelsensor.

Also, when the residual amount of condensed water (calculatedvalue=estimated value) is equal to or smaller than about 0, there is apossibility that the drain valve is excessively opened due to a failureof the water level sensor. Thus, if the hydrogen utilization rate isreduced and hydrogen leakage sensor alarms due to leakage of hydrogeninstead of condensed water, it may be determined that the water levelsensor has failed. Then, condensed water drain valve may be controlledon/off control based on the anode water trap (AWT) estimate to maintainresidual condensed water near the water level sensor.

The methods for controlling anode condensed water drain using a relativehumidity and condensed water estimator according to first and secondembodiments of the present invention will be described in further detailbelow.

First Embodiment

As shown in the flowchart of FIG. 7A, in a first step (S101), a residualamount of condensed water based on water balance in the anode condensedwater collector may be calculated using Equation (1).

$\begin{matrix}{{{Residual}\mspace{14mu}{condensed}\mspace{14mu}{{water}\mspace{14mu}\lbrack{kg}\rbrack}} = {{\sum\limits_{t = 0}^{t = \infty}{\left\lbrack {{AWT} - {\left( {{Anode}\mspace{14mu}{condensed}\mspace{14mu}{water}\mspace{14mu}{drain}\mspace{14mu}{valve}\mspace{14mu}{on}} \right) \times \left( {{condensed}\mspace{14mu}{water}\mspace{11mu}{outflow}\mspace{14mu}{per}\mspace{14mu}{second}} \right)}} \right\rbrack \times \Delta\; t}} + {{Residual}\mspace{14mu}{condensed}\mspace{14mu}{water}\mspace{14mu}{initial}\mspace{14mu}{value}}}} & (1)\end{matrix}$

In a further step (S102), the residual amount of condensed water may becompared with a product of the density (ρ[kg/m{circumflex over ( )}3])of condensed water and the total volume (V1[m{circumflex over ( )}3]) ofthe condensed water collector, and a duration (Δt) may be compared witha reference value (t1).

In a further step (S103), if the residual amount of condensed water isgreater than the product of the density (ρ[kg/m{circumflex over ( )}3])of condensed water and the total volume (V1[m{circumflex over ( )}3]) ofthe condensed water collector, and the duration (Δt) is greater than thereference value (t1), it is determined as a warning stage of a condensedwater level sensor.

As the drain valve is excessively closed due to a failure of the waterlevel sensor, cell voltage drop, abnormal operation of hydrogen recycleblower, and anode stack inlet/outlet pressure abnormality may occur dueto serious flooding in the stack.

Accordingly, if V_cell is smaller than V_cell_TH, ΔV_cell is greaterthan ΔV_cell_TH, hydrogen recycle blower RPM is smaller thanRPM_cmd_RPM_TH, anode stack inlet pressure is greater than inlet normalpressure map plus P_TH, or anode stack outlet pressure is greater thanoutlet normal pressure map plus P_TH, and the duration is greater than areference value t2 in a next step (S104), it may be determined as afailure of the condensed water level sensor in a next step (S105). Here,V_TH, V_cell, V_cell_TH, ΔV_cell, ΔV_cell_TH, RPM_cmd, RPM_TH, and P_THsignify condensed water difference acceptable reference value(m{circumflex over ( )}3), stack cell voltage, stack cell voltage lowestlimit reference value, stack cell voltage deviation, stack cell voltagedeviation upper limit reference value, hydrogen recycle blower RPMcommand value, hydrogen recycle blower RPM difference acceptablereference value, and pressure difference acceptable reference value,respectively.

In a next step (S106), condensed water drain valve control (initialvalue: close) may be initiated based on the anode water trap (AWT)estimated value.

Upon drain valve control, if the residual amount of condensed water isgreater than a product of the density (ρ[kg/m{circumflex over ( )}3]) ofcondensed water and the volume (V2[m{circumflex over ( )}3]) of thecondensed water collector from the bottom thereof to the location of thewater sensor, and the duration (Δt) is greater than a reference value(t3) in a next step (S107), a control for opening the condensed waterdrain valve may be performed in a next step (S108).

On the other hand, if the residual amount of condensed water is smallerthan a product of the density (ρ[kg/m{circumflex over ( )}3]) ofcondensed water and a value obtained by subtracting the condensed waterdifference acceptable reference value (V_TH) from the volume(V2[m{circumflex over ( )}3]) of the condensed water collector from thebottom thereof to the location of the water sensor, or a drain valveopen duration is greater than a reference value (t4) in a next step(S109), a control for closing the condensed water drain valve may beperformed in a next step (S110).

Second Embodiment

As shown in the flowchart of FIG. 7B, the residual amount of condensedwater based on water balance in the anode condensed water collector maybe calculated using Equation (1) in a first step (S201).

Next, if the residual amount of condensed water is equal to or smallerthan about 0, and the duration is greater than a reference value (t5) ina further step (S202), it may be determined as a warning stage of acondensed water level sensor in a next step (S203).

That is, as the drain valve is excessively opened due to a failure ofthe water level sensor, it is determined that vehicle fuel efficiencymay be reduced (reduction of hydrogen utilization rate), and a hydrogenleakage sensor may alarm due to leakage of hydrogen instead of condensedwater.

Accordingly, if calculated hydrogen utilization rate is smaller than avalue obtained by subtracting hydrogen utilization rate acceptablereference value (H2_Util_TH) from normal hydrogen utilization rate map,or the hydrogen leakage sensor is on, and the duration is greater than areference value (t6) in a next step (S204), a failure of the condensedwater level sensor may be determined in a next step (S205).

The normal hydrogen utilization rate map may utilize stack current-basedhydrogen utilization rate test map, and the calculated hydrogenutilization rate may be calculated by Equation (2).

$\begin{matrix}{{{Cuculated}\mspace{14mu}{hydrogen}\mspace{14mu}{{utilization}\mspace{14mu}\lbrack\%\rbrack}} = {\quad{\left\lbrack {\sum\limits_{t = 0}^{t = T}{\left\lbrack {\left( {{stack}\mspace{14mu}{current} \times {stack}\mspace{14mu}{number} \times 0.002} \right) \div \left( {2 \times {Faraday}\mspace{14mu}{constant}} \right)} \right\rbrack \times \Delta\; t}} \right\rbrack \div {\quad\left\lbrack {{{used}\mspace{14mu}{hydrogen}\mspace{14mu}{according}\mspace{14mu}{to}\mspace{14mu}{hydrogen}\mspace{14mu}{tank}\mspace{14mu}{pressure}},{{temperature}\mspace{14mu}\left( {{Van}\mspace{14mu}{der}\mspace{14mu}{Waals}\mspace{14mu}{equation}} \right)}} \right\rbrack}}}} & (2)\end{matrix}$

Accordingly, in a next step (S206), the condensed water drain valvecontrol (initial value: close) may be initiated based on the anode watertrap (AWT) estimated value.

Upon drain valve control, as described in the first embodiment, if theresidual amount of condensed water is greater than the product of thedensity (ρ[kg/m{circumflex over ( )}3]) of condensed water and thevolume (V2[m{circumflex over ( )}3]) of the condensed water collectorfrom the bottom thereof to the location of the water sensor, and theduration (Δt) is greater than the reference value (t3) in a next step(S207), a control for opening the condensed water drain valve may beperformed in a next step (S208).

On the other hand, if the residual amount of condensed water is smallerthan the product of the density (ρ[kg/m{circumflex over ( )}3]) ofcondensed water and the value obtained by subtracting the condensedwater difference acceptable reference value (V_TH) from the volume(V2[m{circumflex over ( )}3]) of the condensed water collector from thebottom thereof to the location of the water sensor, or the drain valveopen duration is greater than the reference value (t4) in a next step(S209), a control for closing the condensed water drain valve may beperformed in a next step (S210).

The methods for controlling anode condensed water drain using a relativehumidity and condensed water estimator according to third and fourthembodiments of the present invention may perform drain valve on/ofcontrol based on the cumulative amount of generated water in the anodecondensed water collector, e.g., a ratio difference of the cumulativeamount of condensed water to the cumulative amount of generated waterunlike those of the first and second embodiments.

The methods for controlling anode condensed water drain according to thethird and fourth embodiments of the present invention illustrativelyincludes determining whether the water level sensor in the condensedwater collector is normal, based on a ratio difference obtained bycalculating a drain valve operation-based cumulative condensed waterratio (AWT_ratio1) based on the cumulative amount of generated water andan estimator-based cumulative condensed water ratio (AWT_ratio2), and ifabnormal, performing drain valve on/off control based a residualcondensed water estimated value.

In this case, when the drain valve operation-based cumulative condensedwater ratio (AWT_ratio1) is smaller than the estimator-based cumulativecondensed water ratio (AWT_ratio2) by an acceptable reference value, thedrain valve may be excessively closed due to a failure of the waterlevel sensor. Thus, if cell voltage drop, abnormal operation of thehydrogen recycle blower, and abnormality of anode-side stackinlet/outlet pressure occur due to serious flooding in the stack, it isdetermined that the water level sensor fails. Then, condensed waterdrain valve on/off control based on the anode water trap (AWT) estimatemay be performed to maintain residual condensed water near the waterlevel sensor.

Also, when the drain valve operation-based cumulative condensed waterratio (AWT_ratio1) is greater than the estimator-based cumulativecondensed water ratio (AWT_ratio2) by the acceptable reference value,the drain valve may be excessively opened due to a failure of the waterlevel sensor. Thus, if the hydrogen utilization rate is reduced, andhydrogen leakage sensor alarms due to leakage of hydrogen instead ofcondensed water, it may be determined that the water level sensor fails.Then, condensed water drain valve on/off control based on the anodewater trap (AWT) estimate may be performed to maintain residualcondensed water near the water level sensor.

Hereinafter, the method for controlling anode condensed water drainusing the relative humidity and condensed water estimator according tothe third and fourth embodiments will be described in further detail.

Third Embodiment

As shown in the flowchart of FIG. 8A, the cumulative amount of condensedwater, the cumulative amount of generated water, and the cumulativecondensed water estimated value, based on the operation of the drainvalve in the anode condensed water collector, may be calculated usingEquations (3), (4), and (5) in a first step (S301).

$\begin{matrix}{{{Cumulative}\mspace{14mu}{condensed}\mspace{14mu}{{water}\mspace{14mu}\lbrack{kg}\rbrack}} = {\sum\limits_{t = 0}^{t = \infty}{\quad\left\lbrack {\left( {{Anode}\mspace{14mu}{condensed}\mspace{14mu}{water}\mspace{14mu}{drain}\mspace{14mu}{valve}\mspace{14mu}{on}} \right) \times \left. \quad\left( {{Condensed}\mspace{14mu}{water}\mspace{14mu}{outflow}\mspace{14mu}{per}\mspace{14mu}{second}} \right) \right\rbrack \times \Delta\; t} \right.}}} & (3) \\{{{Cumulative}\mspace{14mu}{generated}\mspace{14mu}{{water}\mspace{11mu}\lbrack{kg}\rbrack}} = {\sum\limits_{t = 0}^{t = \infty}{\quad\left\lbrack {{\left( {{stack}\mspace{14mu}{current} \times {stack}\mspace{14mu}{number} \times 0.018} \right) \div \left( {2 \times {Faraday}\mspace{11mu}{constant}} \right\rbrack} \times \Delta\; t} \right.}}} & (4) \\{{{Cumulative}\mspace{14mu}{condensed}\mspace{14mu}{water}\mspace{14mu}{{estimate}\mspace{14mu}\lbrack{kg}\rbrack}} = {\sum\limits_{t = 0}^{t = \infty}{\lbrack{AWT}\rbrack \times \Delta\; t}}} & (5)\end{matrix}$

In this case, the drain valve operation-based cumulative condensed waterratio (AWT_ratio1) and the estimator-based cumulative condensed waterratio (AWT_ratio2) may be calculated by Equations (6) and (7) below.

$\begin{matrix}{{AWT\_ ratio1} = {\frac{{cumulative}\mspace{14mu}{condensed}\mspace{14mu}{water}}{{cumulative}\mspace{14mu}{generated}\mspace{14mu}{water}} \times 100\%}} & (6) \\{{{AWT\_ ratio}\; 2} = {\frac{{cumulative}\mspace{14mu}{condensed}\mspace{14mu}{water}\mspace{14mu}{estimate}}{{cumulative}\mspace{14mu}{generated}\mspace{11mu}{water}} \times 100\%}} & (7)\end{matrix}$

If the cumulative condensed water ratio (AWT_ratio1) is smaller than avalue obtained by subtracting the cumulative condensed water ratiodifference acceptable reference value (AWT_TH) from the cumulativecondensed water ratio (AWT_ratio2), and the duration is greater than thereference value t1 in a further step (S302), it may be determined as awarning stage of a condensed water level sensor in a next step (S303).

As the drain valve is excessively closed due to a failure of the waterlevel sensor, cell voltage drop, abnormal operation of hydrogen recycleblower, and anode stack inlet/outlet pressure abnormality may occur dueto serious flooding in the stack.

Accordingly, if V_cell is smaller than V_cell_TH, ΔV_cell is greaterthan ΔV_cell_TH, hydrogen recycle blower RPM is smaller thanRPM_cmd_RPM_TH, anode stack inlet pressure is greater than inlet normalpressure map plus P_TH, or anode stack outlet pressure is greater thanoutlet normal pressure map plus P_TH, and the duration is greater than areference value (t2) in a next step (S304), it may be determined as afailure of the condensed water level sensor in a next step (S305)

Here, V_TH, V_cell, V_cell_TH, ΔV_cell, ΔV_cell_TH, RPM_cmd, RPM_TH, andP_TH signify condensed water difference acceptable reference value(m{circumflex over ( )}3), stack cell voltage, stack cell voltage lowestlimit reference value, stack cell voltage deviation, stack cell voltagedeviation upper limit reference value, hydrogen recycle blower RPMcommand value, hydrogen recycle blower RPM difference acceptablereference value, and pressure difference acceptable reference value,respectively.

In a next step (S306), condensed water drain valve control (initialvalue: close) may be initiated based on the anode water trap (AWT)estimated value.

Upon control of the drain valve, if the residual amount of condensedwater is greater than the product of the density (ρ[kg/m{circumflex over( )}3]) of condensed water and the volume (V2[m{circumflex over ( )}3])of the condensed water collector from the bottom thereof to the locationof the water sensor, and the duration (Δt) is greater than the referencevalue (t3) in a next step (S307), a control for opening the condensedwater drain valve may be performed in a next step (S308).

On the other hand, if the residual amount of condensed water is smallerthan the product of the density (ρ[kg/m{circumflex over ( )}3]) ofcondensed water and the value obtained by subtracting the condensedwater difference acceptable reference value (V_TH) from the volume(V2[m{circumflex over ( )}3]) of the condensed water collector from thebottom thereof to the location of the water sensor, or the drain valveopen duration is greater than the reference value t4 in a next step(S309), a control for closing the condensed water drain valve may beperformed in a next step (S310).

Fourth Embodiment

As shown in the flowchart of FIG. 8B, the cumulative amount of condensedwater, the cumulative amount of generated water, and the cumulativecondensed water estimated value, based on the operation of the drainvalve in the anode condensed water collector, may be calculated usingEquations (2), (3), and (4) of the third embodiment in a first step(S401).

Next, if the cumulative condensed water ratio (AWT_ratio1) is greaterthan a value obtained by adding the cumulative condensed water ratiodifference acceptable reference value (AWT_TH) to the cumulativecondensed water ratio (AWT_ratio2), and the duration is greater than thereference value (t5) in a further step (S402), it may be determined as awarning stage of a condensed water level sensor in a next step (S403).

That is, as the drain valve is excessively opened due to a failure ofthe water level sensor, it is determined that vehicle fuel efficiencymay be reduced (reduction of hydrogen utilization rate), and a hydrogenleakage sensor may alarm due to leakage of hydrogen instead of condensedwater.

Accordingly, if the calculated hydrogen utilization rate is smaller thana value obtained by subtracting hydrogen utilization rate acceptablereference value (H2_Util_TH) from normal hydrogen utilization rate map,or the hydrogen leakage sensor is on, and the duration is greater than areference value t6 in a next step (S404), a failure of the condensedwater level sensor may be determined in a next step (S405).

As described in the second embodiment, the normal hydrogen utilizationrate map may utilize a stack current-based hydrogen utilization ratetest map, and the calculated hydrogen utilization rate may be calculatedby Equation (2).

In a next step (S406), condensed water drain valve control (initialvalue: close) may be initiated based on the anode water trap (AWT)estimated value.

Upon drain valve control, if the residual amount of condensed water isgreater than a product of the density (ρ[kg/m{circumflex over ( )}3]) ofcondensed water and the volume (V2[m{circumflex over ( )}3]) of thecondensed water collector from the bottom thereof to the location of thewater sensor, and the duration (Δt) is greater than a reference value(t3) in a next step (S407), a control for opening the condensed waterdrain valve may be performed in a next step (S408).

On the other hand, if the residual amount of condensed water is smallerthan a product of the density (ρ[kg/m{circumflex over ( )}3]) ofcondensed water and a value obtained by subtracting the condensed waterdifference acceptable reference value (V_TH) from the volume(V2[m{circumflex over ( )}3]) of the condensed water collector from thebottom thereof to the location of the water sensor, or a drain valveopen duration is greater than a reference value t4 in a next step(S409), a control for closing the condensed water drain valve may beperformed in a next step (S410).

Advantageously, an estimator according to an embodiment of the presentinvention can estimate relative humidity and condensed water in a fuelcell system based on typical sensor signals without a separate sensor.Furthermore, the estimator can improve the reliability of controlling acondensed water drain valve in terms of fail-safety, based on thefailure determination of a typical anode condensed water level sensor.In addition, when the water level sensor of the condensed water drainvalve fails, flooding can be prevented in a stack, and a hydrogenrecycle blower unit can be prevented from breaking down. Also, the fuelefficiency and safety can be improved by preventing an excessivehydrogen leakage.

The invention has been described in detail with reference to preferredembodiments thereof. However, it will be appreciated by those skilled inthe art that changes may be made in these embodiments without departingfrom the principles and spirit of the invention, the scope of which isdefined in the appended claims and their equivalents.

What is claimed is:
 1. A method for controlling condensed water drainusing a relative humidity and condensed water estimator for a fuel cell,the method comprising: calculating a residual amount of condensed waterbased on a water balance equation in an anode condensed water collector;determining as a warning stage of a condensed water level sensor if theresidual amount of condensed water is equal to or smaller than 0, and aduration thereof is greater than a reference value (t5); determining asa failure of the condensed water level sensor if a calculated hydrogenutilization rate is smaller than a value obtained by subtracting ahydrogen utilization rate acceptable reference value from a normalhydrogen utilization rate map, or if a hydrogen leakage sensor is on anda duration of an on-state of the hydrogen leakage sensor is greater thana reference value (t6); and if the failure has been determined,performing a control of a condensed water drain valve based on an anodewater trap (AWT) estimated value.
 2. The method of claim 1, wherein theresidual amount of condensed water is calculated by the followingequation:${{Residual}\mspace{14mu}{condensed}\mspace{14mu}{{water}\mspace{14mu}\lbrack{kg}\rbrack}} = {{\sum\limits_{t = 0}^{t = \infty}{\left\lbrack {{AWT} - {\left( {{Anode}\mspace{14mu}{condensed}\mspace{14mu}{water}\mspace{14mu}{drain}\mspace{14mu}{valve}\mspace{14mu}{on}} \right) \times \left( {{condensed}\mspace{14mu}{water}\mspace{14mu}{outflow}\mspace{14mu}{per}\mspace{14mu}{second}} \right)}} \right\rbrack \times \Delta\; t}} + {{Residual}\mspace{14mu}{condensed}\mspace{14mu}{water}\mspace{14mu}{initial}\mspace{14mu}{{value}.}}}$3. The method of claim 1, wherein the normal hydrogen utilization ratemap utilizes a stack current-based hydrogen utilization rate test map,and the calculated hydrogen utilization rate is calculated by followingequation:${{Calculated}\mspace{14mu}{hydrogen}\mspace{14mu}{{utilization}\mspace{14mu}\lbrack\%\rbrack}} = {\quad{\left\lbrack {\sum\limits_{t = 0}^{t = T}{\left\lbrack {\left( {{stack}\mspace{14mu}{current} \times {stack}\mspace{14mu}{number} \times 0.002} \right) \div \left( {2 \times {Faraday}\mspace{14mu}{constant}} \right)} \right\rbrack \times \Delta\; t}} \right\rbrack \div {\quad{\left\lbrack {{{used}\mspace{14mu}{hydrogen}\mspace{14mu}{according}\mspace{14mu}{to}\mspace{14mu}{hydrogen}\mspace{14mu}{tank}\mspace{14mu}{pressure}},{{temperature}\mspace{14mu}\left( {{Van}\mspace{14mu}{der}\mspace{14mu}{Waals}\mspace{14mu}{equation}} \right)}} \right\rbrack.}}}}$4. The method of claim 1, wherein, upon drain valve control, if theresidual amount of condensed water is greater than a product of thedensity (ρ[kg/m{circumflex over ( )}3]) of condensed water and a volume(V2[m{circumflex over ( )}3]) of the condensed water collector from thebottom thereof to a location of the water sensor, and the duration isgreater than a reference value (t3), a control for opening the condensedwater drain valve is performed.
 5. The method of claim 1, wherein, upondrain valve control, if the residual amount of condensed water issmaller than a product of the density (ρ[kg/m{circumflex over ( )}3]) ofcondensed water and a value obtained by subtracting a condensed waterdifference acceptable reference value (V_TH) from a volume(V2[m{circumflex over ( )}3]) of the condensed water collector from thebottom thereof to a location of the water sensor, or a drain valve openduration is greater than a reference value (t4), a control for closingthe condensed water drain valve is performed.